Polyolefin-based films, including those comprising polypropylene, have been known as porous films according to the prior art, but these films lack heat resistance and when used at temperatures exceeding 180° C., for example, such films and their pores undergo considerable dimensional changes, leading to such problems as reduced function as a porous films.
Aromatic polyamides are known as alternative films with excellent heat resistance, and porous films of these polymers are described in Japanese Examined Patent Publication No. 59-14494 and No. 59-36939. In recent years, however, there has been a trend toward thinner porous films which has also led to a higher Young's modulus required for such films; as the current processes have not allowed production of aromatic polyamide porous films with adequate Young's moduli, it has been attempted to improve the Young's modulus through orientation and crystallization achieved by stretching. However, the conventional processes have been associated with problems such as, for example, destruction of the porous structure due to softening at the high stretching temperatures for dry hot stretching, causing densification and closing of the pores, or failure to achieve an increased draw ratio due to the low temperatures of stretching in boiling water, such that the Young's modulus cannot be improved.
On the other hand, several different types of products have been developed as sheets to be used as battery separators for lithium ion batteries and the like, among which synthetic fiber nonwoven fabrics are widely used because of their excellent gas permeability and impregnating ability and satisfactory mechanical properties (for example, Japanese Unexamined Patent Publications No. 63-108664, No. 63-108665 and No. 4-56062). Problems that typically occur with batteries include deformation, extrusion of contents and ignition as a result of overcharging or external shorting, and such problems have been countered by adopting the use of highly heat resistant and chemically resistant aromatic polyamide porous films as separators for batteries, as disclosed in the aforementioned Japanese Unexamined Patent Publications No. 59-14494 and No. 59-36939. In addition, the use of nonwoven fabrics or sheets made of wholly aromatic polyamide (aramide) fibers as battery separators has been proposed in Japanese Unexamined Patent Publications No. 5-335005, No. 7-78608 and No. 7-37571. Nevertheless, while such battery separators exhibit lower internal electrical resistance and more excellent electrical properties with smaller thicknesses, it is very difficult to industrially produce them to thicknesses of 50 μm or smaller with adequate strength and with excellent uniformity, when such films, nonwoven fabrics and sheets as described above are used.
A separator in a battery is an important member situated between the positive electrode and the negative electrode, and recent years have seen modifications designed to provide a shutdown function for separators in order to meet the contradictory requirements of expected electrical properties and safety. Shutdown refers to cutting off the current, due to closing of the pores as the porous film of the separator melts upon increased battery temperature, that occurs in the event of troubles such as overcurrent or external shorting. For this purpose, Japanese Unexamined Patent Publications No. 60-52 and No. 60-136161 have proposed using supports consisting of porous films made of polyethylene or polypropylene that allow manufacture of such thin porous films, and including a low melting point obstructive material capable of melting with heat, such that at high temperatures the obstructive material melts and plugs the pores of the porous film to provide a shutdown function. However, since such materials employ polyethylene or polypropylene thermoplastic films as the base materials, the heat resistance is low and there are many restrictions on their use from a safety standpoint.